Crustal Seismic Velocity Research Articles

Study of crustal seismic velocity in the Weihe fault depression basin by seismic tomographic imaging

This paper gives a study of 3-D crustal seismic velocity changes in the Weihe fault depression basin, in which the overlapping-sphere iterative reconstruction method of tomographic image was used in connection with the two-point fast ray tracing technique. By means of theoretical modeling, the monitoring function of the observatory network system of Shaanxi Province was tested. Using the seismic data of the network, seismic tomographic inversion imaging of the crustal seismic velocity in the Weihe fault basin was studied. The results are as follows: In the Tongchuan-Yaoxian area to the north of Jingyang, there is a high-velocity region extending nearly in NS direction, the highest velocity value is around Tongchuan. To the southwest of Shangxian and Lantian, there is a low-velocity zone about 100 km long and about 50 km wide, inside which there are two regions of the lowest velocity 50 km apart. The epicenters of historical strong earthquakes are mainly on the boundary of high-velocity regions or in regions of fairly high-velocity. In the eastern and western parts of the south margin of the Qinling Mountains, there is an obvious lateral nonhomogeneity of seismic velocity.

Seismic velocity, heterogeneity, and the composition of the lower crust

Many wide-angle seismic experiments attempt to constrain lower crustal seismic velocity and thus composition. Deep near-normal-incidence seismic data recorded during the past decade suggest a widely varying and highly complex pattern of lower crustal heterogeneity. Synthetic seismic modelling indicates that velocities measured from wide-angle experiments may depend significantly upon the geometrical arrangement and scale of the heterogeneities. Heterogeneities with a correlation length considerably large than the seismic wavelength will tend to produce observed velocities biased towards high values. Heterogeneities much smaller than the seismic wavelength can lead to a velocity bias of either sense depending upon their arrangement. Seismic velocities greater than 7.3 km/s measured near the base of the crust in underplated and intruded passive continental margins suggest compositions that are markedly more mafic than mantle melting calculations predict. Our seismic modelling suggests that the mismatch between modelled and observed velocities at passive margins may be attributable to large-scale (correlation length greater than seismic wavelength) heterogeneities.

Stochastic characterization and seismic response of upper and middle crustal rocks based on the Lewisian gneiss complex, Scotland

SUMMARY We have developed statistical models of upper and middle crustal seismic velocity heterogeneity based on geologic maps and petrophysical data from the Lewisian gneiss complex, Scotland. the level of heterogeneity we have measured is relevant to seismic exploration of the crystalline crust using conventional reflection and refraction techniques. We digitized two 1:10560 geologic maps of Laxfordian (Archean) age Lewisian rocks on a 26.8m grid. Both maps are believed to be representative of the upper and middle crust in north-western Scotland, and both are believed to provide cross-sectional views of parts of the crust. the digital maps were characterized by the statistics of their lithologic populations and by their 2-D spatial autocorrelation functions. Different lithologies were assigned seismic velocities appropriate for the mid-crust using petrophysical data. Three lithologies are dominant: silicic gneisses (Vp∼6.2 km s−1), mafic amphibolites (Vp— 6.75 kms−1), and intermediate composition schists (Vp—6.5kms−1). Both maps have self-affine spatial fabrics. The first map covers the core of a syncline. Its autocorrelation function defines a medium with a fractal dimension of 2.78, a horizontal characteristic length of about 244m and a vertical correlation of about 133m (aspect ratio is 2:1). It has an essentially trimodal velocity (lithology) population consisting of 37 per cent silicicgneiss, 43 per cent mafic amphibolites, and 20 per cent schists. This map is representative of 30-40 per cent of Laxfordian rocks. the second map is a plan view which can be rotated 90° about an axis perpendicular to strike to give a cross-section. This map is characterized by a fractal dimension of 2.55, with a horizontal correlation length of about 111m and a vertical correlation of about 38m (aspect ratio 3:1). It has a nearly bimodal population consisting of 77 per cent silicic gneisses and 22 per cent mafic amphibotites. It is representative of 60-70 per cent of Laxfordian rocks. Lastly we examine the seismic response of an upper to middle crust based on our statistical models using acoustic and elastic 2-D finite-difference synthetic seismograms. Short-offset shot records demonstrate that a Lewisian upper crust produces scattered waves which significantly disrupt signals reflecting from deeper levels. Measurements of transmission scattering Q, and coda decay rates confirm that seismic scattering in Lewisian-type crust is strong. the migrated CMP response of a Lewisian crustal model shows the characteristic ‘salt and pepper’ pattern often observed in the upper crust, and described, incorrectly, as ‘transparent'. We suggest that ‘translucent’ is a more appropriate descriptor.

Seismic imaging of oceanic layer 2A between 9°30′N and 10°N on the East Pacific Rise from two‐ship wide‐aperture profiles

We analyze two‐ship multichannel seismic wide aperture profiles (WAP) acquired on the East Pacific Rise (EPR) between 9°30′C and 10°N in May and June 1985. For offsets between 2 and 5 km, most of the WAP common depth point (CDP) gathers exhibit a strong retrograde, reflection‐like seismic arrival that corresponds to rays turned within a very strong velocity gradient zone underlying the uppermost crustal layer 2A. Stacking of this postcritical arrival for hundreds of CDP gathers along the WAP lines yields remarkably clear, and virtually continuous, images of the bottom of layer 2A for several tens of kilometers along and across the EPR. An innovative combination of this image with others of the seafloor (SF) and of the axial magma chamber (AMC) provides an unusually clear composite picture of young upper crust. We combine these images with detailed seismic velocity information, also obtained from the CDP gathers, to achieve an accurate determination of the layer 2A thickness in the area. Along the EPR axis between 9°30°N and 10°N, layer 2A shows a relatively constant thickness between 100 and 150 m (80–120 ms two‐way travel time). Across the axis, however, a line near 9°30′N indicates that layer 2A thickness rapidly increases by approximately a factor of 2 within 2–4 km of the axis and remains nearly constant afterwards. Thus the final thickness of layer 2A is attained before the development of large‐scale tectonic faulting, strongly suggesting that the thickening of layer 2A away from the axis is due to successive episodes of volcanic activity increasing the thickness of the uppermost crustal section from the top, rather than tectonic fracturing lowering the uppermost crustal seismic velocities and pushing the bottom of layer 2A downwards as the crust moves away from the axis.

Oceanic crust thickens approaching the Clipperton Fracture Zone

A multi-channel seismic reflection image shows the reflection Moho dipping toward the Clipperton Fracture Zone in crust 1.4 my old. This seismic line crosses the fracture zone at its eastern intersection with the East Pacific Rise. The seismic observations are made in travel time, not depth. To establish constraints on crustal structure despite the absence of direct velocity determinations in this region, the possible effects of temperature, tectonism, and anomalous lithospheric structure have been considered. Conductive, advective, and frictional heating of the old crust proximal to the ridge-transform intersection can explain <20% of the observed travel-time increase. Heating has a negligible effect on crustal seismic velocity beyond ~10 km from the ridge tip. The transform tectonized zone extends only 6 km from the ridge tip. Serpentinization is unlikely to have thickened the seafloor-to-reflection Moho section in this case. It is concluded that, contrary to conventional wisdom, the 1.4 my old Cocos Plate crust thickens approaching the eastern Clipperton Ridge-Transform Intersection. Increase in thickness must be at least 0.9 km between 22 and 3 km from the fracture zone.

Tomographic image of the axial low‐velocity zone at 12°50′N on the East Pacific Rise

The 1982 MAGMA seismic refraction experiment yielded a large set of accurate P wave travel times corrected for bathymetry and anisotropy which sample the structure of the East Pacific Rise (EPR) at 12°50′N. The arrivals were recorded using ocean bottom seismographs at three sites: on axis, and at 7 km and 16 km east of the axis. We invert 2320 travel times for the twodimensional crustal seismic velocity structure across the EPR axis, assuming that the velocity structure is invariant along strike. The travel times provide strong evidence for compressional velocity anisotropy in the upper crust corresponding to ∼10% faster velocities for propagation parallel to the axis than perpendicular to the axis; the travel tunes used for the tomography are corrected for the effects of this azimuthal anisotropy. Our preferred model contains only the structure clearly required by the data (structure which is stable under excessive smoothing) and achieves a variance reduction of 81% relative to the laterally homogeneous starting model. We resolve a substantial zone beneath the rise axis in which the velocity is reduced by 0.4 to 0.7 km/s; this low‐velocity zone (LVZ) is about 7 km wide and extends from a depth of about 1.5–2.0 km down to Moho at a depth of 5.5 km. The LVZ is slightly asymmetric, extending 1 km further to the east than to the west of the axis. In the shallow (&lt;1.0 km depth) crust, a pattern of velocity variations is imaged in which velocities are high at the spreading axis, decrease between 3 km and 7 km east of the axis, and then increase again between 10 and 15 km east of the axis. We investigate the resolution of the inversion using an impulse response method; the LVZ and off‐axis upper crustal variations are well resolved. In addition, the travel time data indicate that an axial high‐velocity anomaly less than 2 km wide exists in the upper crust but is not resolved by the inversion. The small velocity reductions of the LVZ are consistent with hot rock containing only a small quantity of melt. These results, combined with the multichannel seismic reflection lines and expanded spread profiles from the northern EPR, suggest that the zone of high melt fraction under the spreading center is confined to a narrow, thin lens capping a broad zone of hot plutonic rocks. The upper crustal velocity reduction within 1 km of the axis reflects near‐axial thickening of the extrusive layer and the later reduction probably reflects porosity increases due to near‐axial tectonism; the upper crustal velocity increase beyond 15 km off axis is attributed to porosity decreases associated with hydrothermal alteration.

Heat flow in the Oregon Cascade Range and its correlation with regional gravity, Curie point depths, and geology

New heat flow data for the Oregon Cascade Range are presented and discussed. Heat flow measurements from several deep wells (up to 2500 m deep), as well as extensive new data from industry exploration efforts in the Breitenbush and the Santiam Pass‐Belknap/Foley areas are described. The regional heat flow pattern is similar to that discussed previously. The heat flow is about 100 mW m−2 in the High Cascade Range and at the eastern edge of the Western Cascade Range, It is about 40–50 mW m−2 to the west in the outer arc block of the subduction zone. In the high heat flow zone the heat flow is low at shallow depths in young volcanic rocks due to the high permeability of the rocks and the resultant rapid groundwater flow. Below a depth of 200–400 m much of the area appears to be dominated by conductive heat transfer at least to 2–2.5 km depth. There are perturbations to the regional heat flow in the vicinity of the hot springs where values are up to twice the background. The gravity field in the Cascade Range has characteristics that can be closely related to the heat flow pattern. The relationship may be causal, and to examine the relationship in more detail, earlier two‐dimensional modeling is extended to three dimensions. Consideration of the effects of a midcrustal density anomaly, such as might be associated with a region with at least areas of partial melt, has two major consequences. The first of these is that a high‐frequency gravity gradient near the Western Cascade Range/High Cascade Range boundary is explained. Second, the negative gravity anomaly associated with the north half of the High Cascade Range can be removed, and as a result, the prominent northeast/southwest striking regional Bouguer gravity anomaly associated with the north edge of the Blue Mountains becomes continuous across the Cascade Range with a similar feature along the north side of the Klamath Mountains. Apparently, this zone is a major crustal feature upon which the negative gravity anomaly coincident with the high heat flow is superimposed. The correlation, or lack thereof, of the heat flow, depth to Curie point, gravity field, crustal electrical resistivity, crustal seismic velocity, and geology in the High/Western Cascade Ranges is summarized. Many of the data show aspects that can be interpreted in relation to possible high temperatures in the midcrust of the Cascade Range. The High Cascade Range midcrust has unusually high temperatures and contains a zone of magma staging at 10±2 km depth that can also be identified in subdued form in the Cascade Range in Washington and British Columbia.

The ocean-continent boundary off the western continental margin of Iberia-I. Crustal structure at 40°30'N

SUMMARY The western continental margin of the Iberian peninsula has the characteristics of a rifted non-volcanic margin with half-graben and tilted fault blocks seen in several places. The ocean-continent boundary (OCB) is therefore expected to be where thinned continental crust and oceanic crust are juxtaposed, as at many similar margins worldwide. It is particularly useful to locate the OCB off western Iberia in order to constrain the pre-rift fit of North America to Iberia and, by implication, the shape of the proto-Bay of Biscay. The fit is only marginally constrained by sea-floor spreading magnetic anomalies because anomaly 34 is believed to be far to the west of the OCB and it is even possible that all older oceanic crust was created during the Cretaceous constant polarity interval. The best way to distinguish oceanic crust from thinned continental crust appears to be the crustal seismic velocity structure. Therefore in 1986 a series of seismic refraction lines was shot parallel to, and normal to, the continental margin. These lines enabled us to bracket the location of the OCB. A further constraint on the location was obtained by modelling an east-west magnetic profile which included the enigmatic J-anomaly. This anomaly can be explained as either just pre-anomaly MO or as part of the Cretaceous constant polarity interval, depending on whether spreading began about 127 or after 118 Myr ago, respectively. The evidence favours the former explanation. Lastly the depth to acoustic basement was contoured from a compilation of seismic reflection profiles. This indicated a new fracture zone at 4115'N which offsets the OCB. A few key reflection profiles also suggest that the OCB can be identified by an abrupt landward step-down in acoustic basement. We conclude that the OCB in the eastern Iberia Abyssal Plain lies between 12'10' and 12'30'W and has a trend just east of north. This westerly location is consistent with recent estimates of the location of the OCB off the Grand Banks but brings into question the proposed location at about 11W of the OCB in the Tagus Abyssal Plain.

Lower crustal seismic velocities from Lofoten-Vesterålen, North Norway

The archipelago of Lofoten-Vesterålen comprises Archaean and Proterozoic rocks in granulite or amphibolite facies. The area is characterised by a large gravity anomaly (+120 mGal) and thin crust (c. 25 km) and it has been suggested that it represents a deep section into the continental crust. Compressional and shear wave velocities have been measured in the laboratory on over 70 samples from the area, most of them to 0.4 GPa confining pressure. Velocity profiles are then deduced for sections of the crust assuming a restored horizontal Moho and it is shown that “upper” and “lower” crustal divisions of approximately 5.9–6.2 km s −1 and 6.5–6.6 km s −1 can be recognised, which are similar to those found on seismic refraction lines in Scandinavia. The velocities of the Lofoten granulites are, however, higher than the velocities observed on the Lofoten seismic refraction experiment. Possible reasons for this, including the opening of cracks at low pressure, retrograde metamorphism, the interpretation of the refraction data, or major tectonic dislocation, are explored.

Curie temperature isotherm analysis and tectonic implications of aeromagnetic data from Nevada

Estimates of the depth to the Curie temperature isotherm in Nevada are in accordance with other regional geologic and geophysical information and together can be explained in the context of present‐day tectonism. A method to estimate the depth extent of magnetic sources from the statistical properties of magnetic anomalies was applied to a statewide compilation of aeromagnetic data from Nevada. Basal depths of magnetic sources show no apparent correlation with the so‐called magnetic quiet zone, which trends northerly through the eastern part of the state, or with basin‐and‐range topography. However, certain correlations with published heat flow measurements are apparent and suggest that undulations in basal depth of magnetic sources are related in part to undulations in the Curie temperature isotherm. For example, an area of shallow basal depth (&lt;10 km) near Battle Mountain corresponds to an area of exceptionally high conductive heat flow and indicates a shallow depth to the Curie temperature isotherm in this region. A narrow zone of shallow basal depth extends south from the Battle Mountain area along the 118°W meridian to at least latitude 38°N, which also is a zone of historic surface offsets and high‐magnitude earthquakes. The correspondence along the 118° meridian of shallow basal depth, high heat flow, high lower crustal seismic velocities, attenuated P and S wave arrivals, historic faulting, and large earthquakes suggests that they each are related to an active north trending spreading zone in this part of the Basin and Range province.

Evidence from the northern Red Sea on the transition from continental to oceanic rifting

The northern Red Sea is an active rift in the last stages of continental rifting which is beginning to undergo the transition to oceanic seafloor spreading. The rift can be divided into two distinct regions. The “marginal areas” consist of a series of bathymetric terraces, and as indicated by geophysical data, are underlain by a series of rotated fault blocks. The “axial depression”, which forms the axis of deep water, is a continuous region 10 to 25 km wide near the center of the sea which is characterized by high heat flow, a large gravity minimum and very deformed sediments. Both regions are divided along strike into segments roughly 60 km long by cross-trending transfer zones. The presently active tectonics are characterized by the concentration of extension and deformation in the axial depression and by the emplacement of a number of large intrusions. The intrusions occur at distinct sites located at the intersection of transfer zones with the faults bounding the rotated blocks in the marginal areas, and at deeps located in the axial depression halfway between transfer zones. Although extension is becoming concentrated in the axial depression, geophysical data imply that at least the upper few kilometers of the crust in that region consists of non-magnetic “continental type” material and that this ca'apace has been ruptured by large intrusions only at the deeps. A wide range of crustal seismic velocities has been observed in the northern Red Sea, including velocities normally associated with oceanic crust. The velocity variations appear spatially related to corresponding large variations in basement rock type observed on shore in the Eastern Desert of Egypt. The northern Red Sea is in an intermediate stage between a continental rift valley and a young ocean basin and delineation of its structure and tectonics allows a model to be developed for the evolution of a rifted continental margin. Rifting starts in a series of linked half grabens 50–60 km long and formed over detachment surfaces which probably penetrate no deeper than the mid or lower crust. This style of rifting can accommodate at least the amcunt of extension represented by the Gulf of Suez (35 km). At some greater amount of extension, the rift acquires a more symmetric shape, probably through the development of antithetically faulted blocks in the hanging wall. Extension in this stage is still distributed throughout the rift and is accommodated in the upper crust by rotation of fault blocks. At the stage represented by the northern Red Sea, extension becomes concentrated in an axial depression and begins to be partially accommodated by large intrusions which form deeps. The position of the deeps is controlled by the transfer zones established in the earliest stages of rifting. With further extension, the deeps serve as nucleation points that develop into small, discrete cells of seafloor spreading. These cells grow and coalesce to form a continuous mid-ocean ridge spreading center such as is found in the southern Red Sea.

P-wave crustal structure of the Lake Vänern area, Sweden: EUGENO-S Profile 6

Abstract To investigate the deep structure of the Precambrian basement, airgun shots in Lake Vanern, southern Sweden, were recorded on land by two 100 km lines of MARS stations to the south, two Geostore stations east and west of the lake and a 7 km multichannel reflection array on the Varmlandsnas in the centre of the lake. Modelling of two profiles running roughly N-S, parallel to the major tectonic grain and on either side of the Mylonite Zone, shows variation in crustal seismic velocity structure from east to west. Comparison of these models with near vertical incidence stacked sections from the Varmlandsnas experiment and with an E-W profile suggests continuity of reflecting structures in a N-S direction. This E-W profile shows shallowly west dipping, strongly reflective structures, probably associated with the Mylonite Zone and also a small-scale Moho structure, tentatively interpreted as the western limit of a wedge of overriden older Precambrian crust.

Expanding spread profile at the northern Jan Mayen Ridge

An expanding spread seismic profile at the central northern Jan Mayen Ridge, ESP-5, has yielded a crustal seismic velocity distribution which is similar to observations from the thinned continental crust at the Norwegian continental margin. The profile reveals a post-early Eocene sedimentary sequence, about 1. 5 km thick, overlying 1 km of volcanic extrusives and interbedded sediments. Below, there are about 3 km of pre-opening sediments above the seismic basement. The results indicate that the main ridge block is underlain by a thinned crust, possibly only 13.5 km thick. The results are compatible with a continental nature for the main ridge complex.

Expanding spread profile at the northern Jan Mayen Ridge

An expanding spread seismic profile at the central northern Jan Mayen Ridge, ESP-5, has yielded a crustal seismic velocity distribution which is similar to observations from the thinned continental crust at the Norwegian continental margin. The profile reveals a post-early Eocene sedimentary sequence, about 1. 5 km thick, overlying 1 km of volcanic extrusives and interbedded sediments. Below, there are about 3 km of pre-opening sediments above the seismic basement. The results indicate that the main ridge block is underlain by a thinned crust, possibly only 13.5 km thick. The results are compatible with a continental nature for the main ridge complex.

Crustal and upper mantle structure of the northwestern North Island, New Zealand, from seismic refraction data

The crustal and upper mantle structure of the northwestern North Island of New Zealand is derived from the results of a seismic refraction experiment; shots were fired at the ends and middle of a 575 km-long line extending from Lake Taupo to Cape Reinga. The principal finding from the experiment is that the crust is 25 ± 2 km thick, and is underlain by what is interpreted to be an upper mantle of seismic velocity 7.6 ± 0.1 km s−1, that increases to 7.9 km s−1 at a depth of about 45 km. Crustal seismic velocities vary between 5.3 and 6.36 km s−1 with an average value of 6.04 km s−1. There are close geophysical and geological similarities between the north-western North Island of New Zealand and the Basin and Range province of the western United States. In particular, the conditions of low upper-mantle seismic velocities, thin crust with respect to surface elevation, and high heat-flow (70–100 mW m−2) observed in these two areas can be ascribed to their respective positions behind an active convergent margin for about the past 20 Myr.

Some Preliminary Results from Regional Seismic Profiling in Central Australia

In 1985, the Bureau of Mineral Resources (BMR) recorded a regional deep seismic reflection profile of 420 km length from the Northern Arunta Province to the southern part of the Amadeus Basin (Fig. 1); record lengths were 24 s. An additional 90 km of reflection recording were obtained in the form of cross lines or parallel lines. Experimental reflection recording to moderately large offsets was undertaken to study specific geological or structural problems. An east-west seismic refraction profile of 400 km length was also recorded within the Arunta Complex to determine crustal thickness and seismic velocity variations throughout the crust. Gravity was measured at 333 m intervals along all reflection traverses, and airborne magnetics were recorded along line 1 (Fig. 1).

Crustal structure of Atlantic Fracture Zones - III. The Tydeman fracture zone

Summary. An extensive seismic survey was performed in March 1982 on two different parts of the Tydeman fracture zone. This survey consisted of two separate experiments: a reversed seismic refraction experiment, centred on 36°N, 26°W, using ocean-bottom seismometers and free-floating sonobuoys, which was shot with explosives; and a two-ship multichannel experiment of the expanding spread type, also shot with explosives at approximately 36°N, 23°30°W. These experiments are the first, and to date only, wide-angle seismic surveys of fracture zone crust older than 25 Myr. Slope-intercept and tau-p inversion of the OBS/sonobuoy travel time data indicates low crustal and mantle seismic velocities within the fracture zone. These results are confirmed and refined by a more detailed analysis using raytracing and synthetic seismogram modelling for laterally varying structures, which implies velocities for the upper mantle beneath the fracture zone of 7.3–7.5 km s−1. Similar analysis of a line perpendicular to the fracture zone reveals the transition from normal mantle velocities to the low values found beneath the fracture zone, and suggests that the width of the anomalous region is approximately 12 km. the geometry of the OBS experiment permitted a delay time analysis and a further simple tomographic inversion of the traveltime data, which produced an approximate image of the low velocity mantle region associated with the fracture zone. the width of the region of anomalous crust is the same as the width of the sediment filled trough, which marks the fracture zone location at the line intersection. Analysis of the two-ship seismic data from a site some 240 km to the east, also by synthetic seismogram modelling, gave similar results to the OBS/sonobuoy experiment with a mantle velocity of 7.2 km s−1 detected beneath the crust, which possessed lower than normal seismic velocities. the close seismogram spacing of less than 0.1 km allowed us to transform the dataset into the tau-p domain and enabled us to detect arrivals obscured in the X-T domain by the reflected water wave. Both experiments indicate low crustal seismic velocities within the fracture zone together with unusually low velocities of 7.2–7.5 km s−1 in the upper mantle. Comparison with other fracture zone surveys reveals that the seismic velocity in the upper mantle beneath fracture zones decreases with increasing age. We attribute this to serpentinization of mantle peridotite brought about by hydrothermal circulation to the base of the fractured and faulted crust beneath the fracture zone.

The influence of pore pressure on oceanic crustal seismic velocities

The influence of pore pressure on oceanic crustal seismic velocities

Seismic evidence for hydrothermally altered mantle beneath old crust in the Tydeman fracture zone

To study the structure of old Atlantic fracture zones, seismic refraction profiles were obtained using ocean-bottom seismometers and two-ship multichannel expanding spread experiments on two inactive sections of the Tydeman fracture zone. The results indicate that: (a) crustal seismic velocities are low compared with “normal” oceanic crust; (b) there is a marked absence of an identifiable layer 3; (c) the depth to the Moho is shallow, though not exceptionally so; and (d) anomalously low seismic velocities of 7.2–7.5 km s −1 are observed over ranges at which upper mantle velocities are normally detected. S-wave arrivals, corresponding to both crustal and mantle refractions, are also observed, which enable the first estimates of Poisson's ratio within a fracture zone to be made. Using these results, and those from other surveys, we show that the seismic velocity in the upper mantle beneath fracture zones decreases systematically with increasing age. These findings suggest that hydrothermal circulation, which can lower the upper mantle velocity by serpentinization, persists under fracture zones for at least 70 Ma, and occurs at greater depths than for normal oceanic crust. The serpentinization process is not sufficiently rapid to obscure the location of the original crust-mantle boundary.

Australian seismic refraction results, isostasy and altitude anomalies

Although there have been extensive seismic refraction measurements over areas of continental crust, the causes of variation in crustal thickness, and of variation in mean crustal seismic velocity, are poorly understood. If there is regional (3° × 3°) isostatic equilibrium of the crust, such as suggested by many studies of regional gravity, then crustal thickness variation should be caused largely by variation in surface altitude and crustal density. Seismic refraction results from Australia are used here to confirm that these crustal isostatic effects are a major (40%) cause of variation of crustal thickness even in a continent of little surface relief. However, there is an equally important regional variation in crustal thickness unrelated to crustal isostasy—this is thought to be due to density variation in the lower lithosphere1.